Robots are commonly used in the assembly of mechanical parts as well as certain tooling operations. Robots are driven by precision gears and joints which must be protected from excessive mechanical stress in order to ensure a long service life. Most tooling operations generate a certain amount of torque. Some tooling operations, such as a riveting operation, also generate axial and eccentric loads on the tool. A robot operating a tool must be isolated from those loads to the greatest extent possible. In order to isolate a robot from the axial, eccentric and moment loads generated while operating a tool such as a riveting tool, it is necessary to release the tool from a rigid connection with the robot during the rivet upsetting operation. After rivet upsetting is complete, however, it is necessary to return the riveting tool to a rigid connection with the robot in a predetermined home position so that the tool can be accurately relocated to the next riveting coordinates. Since even large rivets are generally less than one centimetre in diameter, extreme accuracy in positioning the riveting tool is required. Such accuracy can only be achieved if the compliance device is capable of repeatedly relocating the tool to a precise predetermined home position.
Many compliance devices have been designed and constructed using metallic springs to absorb tool reaction loads and to return a tool to a home position after a tool operation. While such compliance devices have proven suitable for light duty work, observation has shown that they do not function effectively for heavy tool operations such as riveting. Springs are also undesirable because they fatigue and therefore require frequent adjustment when used in heavy tool applications.
Compliance devices which use fluid springs have also been invented. U.S. Pat. No. 4,669,192 to Matheson et al. entitled Variable Compliance Device teaches a variable compliance control assembly for use in automatic assembly operations. The variable compliance control assembly includes a housing having an interior chamber and a base member positioned on the surface of the interior chamber. The base member is movable in multidirectional fashion. A fluid spring controls the movement of the base member and returns the base member to a normal, self-centering seated position after a robot operation. A disadvantage of this device is that while it isolates a robot from axial and eccentric loads, it does not isolate the robot from moment loads because a fixed pin which rides in a groove in the base member prevents rotational movement of the base member. The device therefore appears to be unsuitable for heavy tool applications such as riveting that can generate significant moment loads when a rivet is upset by a riveting tool.
It is an object of the invention to provide a robust compliance device a robot that isolates the robot from a substantial part of the loads induced by heavy tool operations and reliably returns a robot tool to a predefined home position after a tool operation is completed.
It is a particular object of the invention to provide a compliance device for a robot which is suitable for use in automated riveting operations.